DNA structure Flashcards
Describe the B form of DNA
This structure is the normal DNA in the nucleus
It has 10 base pairs per helical turn
The base pairs are on the inside and the sugar and phosphate are on the outside
Righ-handed turns
Describe the A form of DNA
Morris Wilkins was able to produce the A form
Also a right-handed DNA helix
More tightly wound than B where the size of the major and minor groove is really close and more similarly sized
Describe the Z form of DNA
A left-handed helix
Can be found inside cells but its function is not well established
What is a nucleotide and how are they linked together?
Nucleotides are repeating base, sugar and phosphate units
They are linked together by 3’-5’ phosphate links
Which bases are pyrimidine and which bases are purines?
Cytosine, thymine and uracil are pyrimidines
Adenine and guanine are purines
Pyrimidines consist of a six membered ring
Purines have two rings
How many hydrogen bonds are there between each base pair?
There are three hydrogen bonds between cytosine and guanine
There are two hydrogen bonds between adenine and thymine
What type of helix is tRNA folded in?
A-form
List three unusual DNA structures
- Left-handed or Z’ DNA formed by 5’…GCGCGCGCGC… or 5’…GTGTGTGTGTG
- Four-stranded junction- Holliday junction
- Tetraplex DNA: 4-stranded DNA helix formed at telomeres; involves G-rich DNA sequences
How does a Holliday junction occur?
A Holliday junction arises naturally in living cells through DNA strand exchange between two homologous chromosomes
They have an important role in exchanging genetic information and DNA repair
(look up more)
How do the structural levels of proteins apply to DNA?
Primary- the sequence of bases (determined through DNA sequencing
Secondary- the DNA’s helical structure (X-ray and chemistry)
Tertiary- DNA supercoiling formed naturally in bacteria (electron microscopy)
Quaternary- interlocked chromosomes
Recall the steps of Sanger sequencing
A DNA strand is copied with a DNA polymerase in the presence of inhibitors that arrest DNA synthesis specifically at A, C, G or T
The DNA is denatured and then the strands are separated by length on a polyacrylamide gel
If the DNA or incorporated inhibitor is radioactive or fluorescent, the DNA bands can be visualised and the sequence read
(Between 770-1000 bases can be visualised per read)
What causes DNA supercoiling in bacteria?
DNA gyrase
What causes DNA relaxation in bacteria?
Topo IV and Topo I
What does an E.coli chromosome look like
E.coli chromosome is circular and organised into ~50 independently supercoiled domains
What is chromatin?
DNA is complexed with histones and other proteins to form a nucleoprotein complex called chromatin
Nucleosomes are the basic building blocks of chromatin
List at least three ways mutations arise in DNA
- Spontaneous e.g. loss of bases or hydrolysis of C to U Chemicals and radicals generated by oxidative metabolism:
- Hydroxyl radicals can convert guanine to 8-oxoguanine; a lethal mutagenic base
- Changes base structure
- Insertion between bases (intercalators such as doxorubicin widely used as anticancer drugs)
- Radiation
- UV light produces thymine dimers
- Ionising radiation (X-rays, gamma rays) break DNA chromosomes to cause leukaemia
What happens if something breaks the DNA phosphodiester bonds
You get what is called a translocation where pieces from two different chromosomes are released and they can be linked together
What happens to patients with xeroderma pigmentosum?
They have a defect in excision repair that deals with UV damage to DNA and so are very prone to skin cancer
